Damage,plasticity and failure of ceramics and cementitious composites subjected to multi-axial state of stress

Experimental data on mechanical behavior of ceramics and cementitious composites subjected to triaxial state of stress and verification of the theoretical model capable to describe deformability and fracture of brittle rock-like materials are presented in the paper. To check the validity of the theoretical model the stress–strain curves and stresses at material fracture determined experimentally for brick and mortar were compared with the theoretical predictions. The limit surface at material fracture obtained experimentally from triaxial tests was used in numerical analysis of masonry specimens subjected to compressive loading. These numerical results obtained by employing the Finite Element Method software package Mafem3D were compared with experimental data available in the literature. Fairly good agreement of numerical predictions with experimental results for masonry specimens was observed.     

混凝土楔入劈拉和三点弯曲断裂过程的细观数值模拟

基于离散元思想和Voronoi单元划分技术,利用混凝土细观刚体弹簧元模型,开展了混凝土楔入劈拉试件和三点弯曲切口梁的断裂过程数值仿真分析。从裂缝开展过程、试件破坏形态、荷载-张口位移曲线(P-CMOD)和断裂能等方面,将数值分析结果与已有的试验结果进行了对比。结果表明,细观刚体弹簧元法较准确地模拟了混凝土楔入劈拉试件和三点弯曲切口梁断裂过程。最后,分析了缝高比和骨料体积含量对混凝土断裂过程的影响规律,发现断裂能随骨料体积含量呈单调递增趋势。     

线性规律功能梯度材料断裂行为实验研究

焦散线方法与高速摄影技术结合,对线性规律功能梯度材料的I型静、动态断裂特性进行实验研究,并利用有限元软件进行数值模拟. 首先推导了线性规律功能梯度材料静、动态裂尖的焦散线初始曲线方程和参数方程,提出通过迭代方法根据功能梯度材料的复杂焦散线方程求解应力强度因子. 为对比材料梯度对断裂行为的影响,利用重力沉降法制备了3种梯度变化规律的试件. 然后对功能梯度材料试件进行三点弯曲实验,通过CCD和高速摄影装置采集试件焦散线图像,提取各种静、动态断裂参数. 最后通过与有限元模拟结果进行对比,证明在功能梯度材料断裂实验中焦散线方法的有效性,并详细分析了不同梯度变化规律对材料静、动态断裂性能的影响.      

Experimental and Numerical Study of Shear Fracture in Brittle Materials with Interference of Initial Double Cracks

A simultaneous experimental and numerical study of shear fracture of concrete-like materials is carried out using Brazilian disc specimens with initial double edge cracks and fourpoint bending beam specimens with double edge-notches.The interference effects of two cracks/notches are investigated through varied ligament angles and crack lengths.It is shown that shear fracturing paths change remarkably with the initial ligament angles and crack lengths.The cracked specimens are numerically simulated by an indirect boundary element method.A comparison between the numerical results and the experimental ones shows good agreement.     

冲击载荷下镁铝合金裂纹动态扩展过程的数值模拟

采用基于黏聚裂纹模型的扩展有限元方法,开展了镁铝合金结构冲击破坏过程的数值模拟研究。通过镁铝合金三点弯曲试样冲击实验,获得了不同子弹撞击速度下试样的冲击破坏模式。在此基础上,建立了实验结构的扩展有限元模型,并采用最大主应力准则,以及含损伤型的本构关系模拟材料的冲击断裂行为。对于裂纹尖端附近区域,采用黏聚裂纹模型模拟裂纹的断裂过程。对子弹速度分别为12.2、15.1、26.3 m/s的3种工况下镁铝合金试样的动态破坏过程进行了数值模拟研究,获得了与实验相一致的断裂模式。计算结果表明,试样以Ⅰ型断裂模式为主,裂纹沿初始预制裂纹方向扩展。当裂纹扩展到一定程度后,在试样韧带区域被撞击端附近,由于应力波及边界效应导致该区域处于复杂应力状态,试样出现复合型断裂模式,裂纹偏离原扩展路径,与本文实验结果相吻合。     

Mode I Fracture Characterization of Bituminous Paving Mixtures at Intermediate Service Temperatures

This study presents an integrated approach combining experimental tests and numerical modeling to characterize mode I fracture behavior of bituminous paving mixtures subjected to a wide range of loading rates at intermediate temperature conditions. A simple experimental protocol is developed using the semi-circular bending (SCB) test geometry. The local fracture behavior at the initial notch tip of the SCB specimens is monitored using high-speed cameras with a digital image correlation (DIC) system. The DIC results of the SCB fracture tests are then simulated using a finite element method that is incorporated with material viscoelasticity and cohesive zone fracture. Fracture properties are obtained locally at the notch tip by identifying two cohesive zone fracture parameters (cohesive strength and fracture energy) that result in a good agreement between test results and numerical simulations. The results clearly present significant rate-dependent fracture characteristics of bituminous paving mixtures at intermediate service temperatures. This study further demonstrates that fracture properties of viscoelastic materials need to be characterized at the local fracture process zone when they present ductile fracture behavior.     

In-plane fracture of laminated fiber reinforced composites with varying fracture resistance: Experimental observations and numerical crack propagation simulations

A series of experimental results on the in-plane fracture of a fiber reinforced laminated composite panel is analyzed using the variational multiscale cohesive method (VMCM). The VMCM results demonstrate the influence of specimen geometry and load distribution on the propagation of large scale bridging cracks in the fiber reinforced panel. Experimentally observed variation in fracture resistance is substantiated numerically by comparing the experimental and VMCM load–displacement responses of geometrically scaled single edge-notch three point bend (SETB) specimens. The results elucidate the size dependence of the traction-separation relationship for this class of materials even in moderately large specimens, contrary to the conventional understanding of it being a material property. The existence of a “free bridging zone” (different from the conventional “full bridging zone”) is recognized, and its influence on the evolving fracture resistance is discussed. The numerical simulations and ensuing bridging zone evolution analysis demonstrates the versatility of VMCM in objectively simulating progressive crack propagation, compared against conventional numerical schemes like traditional cohesive zone modeling, which require a priori knowledge of the crack path.     

高导无氧铜杆高应变率拉伸下的颈缩与断裂

采用Hopkinson拉伸实验装置和一种高速拉伸断裂实验新装置,对高导无氧铜(OFHC)杆件进行了一系列高应变率拉伸断裂试验.实验结果表明,局部化的断裂应变随拉伸速度增大并不明显增大,其断裂位置有随机性.存在一种临界拉伸速度,当冲击拉伸速度大于此值时,断裂即发生在冲击拉伸端附近,杆的其它部分几乎无应变.采用典型的Johnson-Cook本构关系,使用LS-DYNA程序进行一系列数值模拟,提出颈缩处直径收缩率达极值的颈缩失效判据,由此计算所得的局部化颈缩应变及断裂位置与试验回收结果有一定差别.     

岩石断裂尺寸效应测试装置研制及实验研究

面向岩石断裂尺寸效应研究的实验装置需求,针对现有技术中三点弯曲装置对多组尺寸岩石试件适应性差、最小跨距的测试量程不足等问题,研制了一种可灵活用于岩石断裂尺寸效应测试的三点弯曲装里.装置采用"两体分离式"的设计,三种不同型号的滚子与压头体、支座体配合使用,有效避免了不同尺寸试件采用同一直径滚子测试带来的实验精度问题;同时...     

On the influence of stress triaxiality and strain rate on the behaviour of a structural steel. Part II. Numerical study

Non-linear finite element analyses of quasi-static and high-rate tensile tests with smooth and notched axisymmetric specimens of the structural steel Weldox 460 E have been carried out. The constitutive relation and fracture criterion of Johnson and Cook, which were adopted in the simulations, have previously been determined for Weldox 460 E steel. First, a validation study was completed to assess the accuracy of the constitutive relation and fracture criterion. The numerical results were compared with experimental data from tensile tests under quasi-static and dynamic loading conditions. Secondly, the use of Bridgman's analysis in the identification of the fracture criterion was evaluated, and the influence of adiabatic heating and inertia on the stress triaxiality in the tensile specimens was investigated. The results were finally used to discuss the identification of fracture criteria based on tensile tests with smooth and notched axisymmetric specimens.     

Hybrid experimental–numerical analysis of basic ductile fracture experiments for sheet metals

A basic ductile fracture testing program is carried out on specimens extracted from TRIP780 steel sheets including tensile specimens with a central hole and circular notches. In addition, equi-biaxial punch tests are performed. The surface strain fields are measured using two- and three-dimensional digital image correlation. Due to the localization of plastic deformation during the testing of the tensile specimens, finite element simulations are performed of each test to obtain the stress and strain histories at the material point where fracture initiates. Error estimates are made based on the differences between the predicted and measured local strains. The results from the testing of tensile specimens with a central hole as well as from punch tests show that equivalent strains of more than 0.8 can be achieved at approximately constant stress triaxialities to fracture of about 0.3 and 0.66, respectively. The error analysis demonstrates that both the equivalent plastic strain and the stress triaxiality are very sensitive to uncertainties in the experimental measurements and the numerical model assumptions. The results from computations with very fine solid element meshes agree well with the experiments when the strain hardening is identified from experiments up to very large strains.     

混凝土楔入劈拉和三点弯曲断裂过程的细观数值模拟Mesoscale simulation on fracture process of wedge splitting and three-point bending of concrete

基于离散元思想和Voronoi单元划分技术,利用混凝土细观刚体弹簧元模型,开展了混凝土楔入劈拉试件和三点弯曲切口梁的断裂过程数值仿真分析。从裂缝开展过程、试件破坏形态、荷载-张口位移曲线(P-CMOD)和断裂能等方面,将数值分析结果与已有的试验结果进行了对比。结果表明,细观刚体弹簧元法较准确地模拟了混凝土楔入劈拉试件和三点弯曲切口梁断裂过程。最后,分析了缝高比和骨料体积含量对混凝土断裂过程的影响规律,发现断裂能随骨料体积含量呈单调递增趋势。     

应力三轴度的有限元计算修正

对某高强度钢制成的光滑圆棒和缺口圆棒进行了系列准静态拉伸实验,采用ABAQUS对每个试 件进行了数值模拟,得到了该材料的真实应力应变曲线,拟合出了J-C本构模型和失效模型的部分材料常数。 最后,对该高强度钢制成的平板进行了撞击实验,并用得到的J-C模型对平板撞击实验进行了数值模拟,计算 结果与实验结果吻合很好,证明利用数值模拟并修正应力三轴度的方法是可行的。     

Plasticity and fracture of martensitic boron steel under plane stress conditions

Two series of multiaxial experiments are performed to characterize the mechanical behavior of a hot formed martensitic 22MnB5 boron steel. In the first series, flat specimens of uniform cross-section are subjected to various combinations of tensile and shear loading to characterize the elasto-plastic response. Butterfly-shaped specimens of non-uniform cross-section are used for the second series to study the onset of fracture in the martensitic steel. It is found from the analysis of the experimental results that the planar isotropic Hill’48 yield function along with an associated flow rule provides good estimates of the stress–strain response over a wide range of loading paths. The fracture experiments demonstrate that the crack initiation depends strongly on the loading state. A simple stress triaxiality dependent phenomenological fracture model is calibrated to describe the onset of fracture. Using the proposed plasticity and fracture model, numerical simulations of the fracture of tensile specimens of different notch radii are performed and compared with experiments.     

三维编织CMC的弯曲断裂研究

从界面断裂的角度出发，对三维编织CMC的断裂作了理论研究和数值分析，对于三点弯曲试件，通过数值拟合修正了能量释放率G的理论表达式中的自由常数A，同时也研究了材料的各个参变量对于断裂韧性的影响，由此得出了一个基本完善的三点弯曲试件断裂韧性G的理论公式，该能量释放率方法可以应用于单试件的试验计算，与断裂韧性的柔度标定方法相比，该方法一方面可以减少试验件数量；另一方面，试验结果显示出在试件切口尺雨处于0．4≤a/W≤0.5时，可以获得比较稳定的断裂韧性值。     

Estimating the tensile strength of super hard brittle materials using truncated spheroidal specimens

New approaches need to be introduced to measure the tensile capacity of super hard materials since the standard methods are not effective. To pursue this objective, a series of laboratory tests were constructed to replicate the fracture mechanism of diamond-based materials. Experiments indicate that under a certain compressive test condition, stresses normal to the axisymmetric line in truncated spheroidal specimens (bullet-shaped specimens) are in tension contributing to the tensile fracture of the material. From experimental and numerical studies, it is concluded that semi-prolate spheroidal specimens can be used to determine precisely the tensile strength of brittle stiff diamond-like composites.     

A combined experimental–numerical investigation of ductile fracture in bending of a class of ferritic–martensitic steel

We present a combined experimental–numerical study on fracture initiation at the convex surface and its propagation during bending of a class of ferritic–martensitic steel. On the experimental side, so-called free bending experiments are conducted on DP1000 steel sheets until fracture, realizing optical and scanning electron microscopy analyses on the post mortem specimens for fracture characterization. A blended Mode I – Mode II fracture pattern, which is driven by cavitation at non-metallic inclusions as well as martensitic islands and resultant softening-based intense strain localization, is observed. Phenomena like crack zig-zagging and crack alternation at the bend apex along the bending axis are introduced and discussed. On the numerical side, based on this physical motivation, the process is simulated in 2D plane strain and 3D, using Gurson’s dilatant plasticity model with a recent shear modification, strain-based void nucleation, and coalescence effects. The effect of certain material parameters (initial porosity, damage at coalescence and failure, shear modification term, etc.), plane strain constraint and mesh size on the localization and the fracture behavior are investigated in detail.     

A micromechanics-based strain gradient damage model for fracture prediction of brittle materials – Part II: Damage modeling and numerical simulations

In this paper, we established a strain-gradient damage model based on microcrack analysis for brittle materials. In order to construct a damage-evolution law including the strain-gradient effect, we proposed a resistance curve for microcrack growth before damage localization. By introducing this resistance curve into the strain-gradient constitutive law established in the first part of this work (Li, 2011), we obtained an energy potential that is capable to describe the evolution of damage during the loading. This damage model was furthermore implemented into a finite element code. By using this numerical tool, we carried out detailed numerical simulations on different specimens in order to assess the fracture process in brittle materials. The numerical results were compared with previous experimental results. From these studies, we can conclude that the strain gradient plays an important role in predicting fractures due to singular or non-singular stress concentrations and in assessing the size effect observed in experimental studies. Moreover, the self-regularization characteristic of the present damage model makes the numerical simulations insensitive to finite-element meshing. We believe that it can be utilized in fracture predictions for brittle or quasi-brittle materials in engineering applications.     

Experimental determination of damage parameters in uniaxially-loaded metal-matrix composites using the overall approach

A recently developed damage-plasticity model for metal matrix composites is examined using the overall approach to damage. In this approach, the damage mechanisms in the composite system are reflected through a single overall damage tensor assuming an elasto-plastic matrix with elastic fibers. It is shown how the model is applied to uniaxially loaded metal-matrix specimens. Each specimen is a symmetric laminate composed of four plies with different orientations. A numerical procedure is formulated and implemented to calculate the stresses, strains, and damage variables in these specimens.

In order to test the validity of the model, a series of uniaxial test experiments are conducted for the damage characterization of the metal-matrix composite. The experiments are conducted on a titanium aluminide SiC-reinforced laminate. Laminate layups of (0/90)s and (± 45)s are tested under uniaxial tension. In order to investigate and model damage evolution, each specimen is loaded to different load levels ranging from the fracture load to 70% of fracture load. Scanning electron microscopy is performed on representative cross-sections of all specimens. This information is then used to quantify damage in the composite system. Damage variables are defined based on the crack density and correlated with the theoretical model. The experimental and numerical results are then compared. Certain recommendations are finally made concerning the determination of damage parameters in metal-matrix composites.     

Dual-parameter elastic-plastic fracture criterion of metals and alloys in the plane strain case

In recent years, some investigators discussed the applicability of the HRR theory for engineering materials based on the results of numerical analyses and experimental studies. In the present paper, the finite element method is employed to analyze the crack tip fields of the engineering elastic-plastic material with a variety of geometry configurations of cracked specimens from elastic state to intensely general yielded state in the plane strain case. The results indicate that the HRR theory loses its validity of application for engineering elastic-plastic materials in the plane strain case. The reasons for this are analyzed. A dual-parameter fracture criterion is suggested for this case.